optimum location of side reboilers , especially in case of demethanizer tower in NGL recovery plant

[mohamedalaa]

hi guys,
currently, i am working on simulation model for NGL recovery. the scheme that i used is GSP (gas subcooled process) and i am confused about the parameters that i have to consider to figure out the best location and number of side reboilers. besides, how much the duty of side reboilers contributes to the total required duty? and if there any common practice for delta T that i have to consider for side reboilers?

 

[MJCronin]

Moe....

The design of distillation columns and reboilers is almost a specialty unto itself.

Thermal optimization, sizing and equipment layout must all work hand in hand. Some simulation software does a great job providing results. Some not so much .... Which simulation package are you using ?

(My recollection is that ChemCad and HYSIS did an adequate job)

There are many threads on this forum discussing reboiler design . Have you found any of them helpful ?

https://www.eng-tips.com/viewthread.cfm?qid=213881

Have you found a discussion forum promoted by the software package (that you purchased) discussing these design questions ?

MJCronin
Sr. Process Engineer

 

[mohamedalaa]

MJCronin

thank you for your help. i am using Aspen hysys for simulation porpuse.I already got the benefits of using the side reboilers to cool down the feed and reduce the refrigeration duty, i just need for some guidelines to decide about the following:
-on which trays i can attach the side reboilers to the demethanizer without affecting on the performance of the tower ?

-Is there a maximum number of side reboilers i can attach to the demethanizer?

-How much i can heat up the side or total draw from the tower in the side reboiler before entering back to the tower?

attached hand sketch to show what i mean.

https://files.engineering.com/getfi...b29&file=15618704462655806064555894748237.jpg

https://files.engineering.com/getfi...45b&file=15618706990672723429791592659750.jpg

thanks for your help in advance

 

[georgeverghese]

In your sketch, you have 3 side reboilers ( from T7 to T8, from T10 to T11 and from T13 to T14). Trying to get a reboiler to manage the flows with the dp across a single theoretical stage may be okay, if the tray eff is say about 30% or so, since this will translate to the exit from the side reboiler returning some 3-4 actual trays below the side draw to enable sufficient dp. But if your tray eff is 60%, then you may have much less dp to work with for the side reboiler. This should be taken into account, and required some detailed knowledge of pass 2phase pressure drop in the cold box multicell HX.

Adding more side reboilers, though it would reduce recompression duty, adds much more complexity. Also, more cold energy recovery makes startup more difficult - I wouldnt allow side reboil duty to be more than 50% of total reboil duty. Other info I see on the internet seems to show 2 side reboilers for the GSP process (excluding bottoms reboiler), which appears onerous enough. The old ExxonMobil demethaniser I worked on many years ago had only 1 side reboiler.

Many of the problems with these plants is with generating sufficient reflux to keep C3 and C2 recovery ratios high in the later years when feed gas composition is leaner than in the early years - you may get better results for the installed CAPEX and OPEX by taking a closer look at the overheads also.

In these multicell plate heat exchangers, a cold side temp approach of about 2-3degC should be possible. In air separation plants, pass cold side dT can be as low as 1degC.